Vigorous research activities are being directed to fuel cells as a next-generation energy source, due to their high generation efficiency and low burden on the environment.
A fuel cell is a power generation device in which hydrogen as a fuel and oxygen are allowed to undergo an electrochemical reaction, from which electric energy is elicited. Depending on the type of electrolyte used, fuel cells are classified into: the Solid Oxide Fuel Cell (SOFC); the Molten Carbonate Fuel Cell (MCFC); the Phosphoric Acid Fuel Cell (PAFC); the Polymer Electrolyte Fuel Cell (PEFC); and the Direct Methanol Fuel Cell (DMFC). Among others, PEFCs and DMFCs have operating temperatures as low as about 70 to 90° C. (as compared with other types of fuel cells), and enable a highly-efficient power generation even at about 1 kW (PEFC) and about several hundred W (DMFC). Therefore, PEFCs and DMFCs are considered especially promising for applications such as automobiles, mobile devices, and the like. In particular, DMFCs are small-sized, and their applications in mobile devices are being vigorously studied.
Hereinafter, with reference to FIG. 3, the structure and principle of a polymer electrolyte fuel cell (PEFC) will be described as an example.
FIG. 3(a) is a perspective view schematically showing the structure of a cell (battery) portion, which is a minimum structural unit of a polymer electrolyte fuel cell (PEFC). FIG. 3(b) is a schematic diagram showing the principle behind a PEFC.
As shown in FIG. 3(a), a cell in a fuel cell includes an ion exchange membrane (solid polymer membrane) 11 in the middle, with two electrodes being disposed on its sides: a fuel electrode (hydrogen electrode, anode side) 12 and an air electrode (or oxygen electrode, cathode side) 13. The ion exchange membrane 11 is a membrane for allowing protons (H+) to move from the fuel electrode 12 to the air electrode 13. It is often the case that the ion exchange membrane 11 has electrode catalyst layers 14a and 14b on both sides thereof, and thus the ion exchange membrane 11 and the electrode catalyst layers 14a and 14b are collectively referred to as a membrane electrode assembly (MEA) 20. On the outside of the fuel electrode 12 and the air electrode 13, separators 16a and 16b are provided via gaskets 15a and 15b, respectively. Thus, hydrogen (anode side) moves between the MEA 20 and the separator 16a, and oxygen (cathode side) moves between the MEA 20 and the separator 16b (see FIG. 3(b)). On the surface of the separators 16a and 16b, trenches are formed through which reaction gases of hydrogen and oxygen pass.
As shown in FIG. 3(b), at the anode side, hydrogen (H2) is supplied through the trench in the surface of the separator 16a, and uniformly diffused into the electrode catalyst layer 14a by the action of the fuel electrode 12. On the electrode catalyst layer 14a, H2 becomes H+ through a reaction of formula (1) below, and passes through the ion exchange membrane 11 and moves to the electrode catalyst layer 14b on the cathode side. On the other hand, at the cathode side, oxygen (O2) is supplied through the trench in the surface of the separator 16b, and uniformly diffused into the electrode catalyst layer 14b by the action of the air electrode 13. On the electrode catalyst layer 14b, a reaction of formula (2) below occurs between the O2 having been diffused in this manner and the H+ which has moved through the ion exchange membrane 11 from the anode side, whereby H2O is generated.H2→2H+2e−. . .   formula (1)2H++½O2+2e−→H2O . . .   formula (2)
At this time, power generation occurs due to the electrons (e−) which are generated at the anode side. Therefore, the separator is required to efficiently supply reaction gases of oxygen and hydrogen to the electrode catalyst layer 14a. 
Depending on the amount of electric power, a plurality of cells (unit cells) having the above construction may be layered so as to be used in the form of a stack. In this case, the separators will act as partitions between unit cells, and therefore are required to ensure that the gas (hydrogen) from the fuel electrode and the gas (oxygen) from the air electrode will not become mixed in between cells.
From these standpoints, a separator is required to have little gas permeability, a good electrical conductivity, a low contact resistance, a good anticorrosiveness, and so on. In particular, there emerge stronger and stronger anticorrosiveness and electrical conductivity requirements in the recent years. As an evaluation criterion of anticorrosiveness, it is proposed that “no rust should occur even if the separator is immersed for 1000 hours in a sulfuric acid solution whose pH is about 1”. In particular, DMFCs are small-sized and therefore are required to have a good surface electrical conductivity.
As a separator material having such characteristics, carbon materials are generally used. However, carbon materials have poor toughness and are brittle, and therefore are difficult to process, thus resulting in a problem of high processing costs.
Therefore, instead of carbon materials, use of metal materials as separator materials has been proposed in the recent years, because metal materials are easy to process and incur low processing costs (Patent Documents 1 to 2).
Above all, Patent Document 1 discloses a separator for a polymer electrolyte fuel cell, in which an electrically conductive ceramic coating such as niobium nitride, molybdenum silicide, tantalum carbide, or the like is provided on the surface of a stainless steel or the like. In Patent Document 1, in order to enhance the electrical conductivity in the operating environment of a fuel cell (i.e., a temperature range from room temperature to near 150° C., vapor atmosphere), an electrically conductive ceramic having a low resistivity is used.
Patent Document 2 discloses a separator for a polymer electrolyte fuel cell, in which a conductive film is formed on the surface of a metal substrate. The Cr concentration at the surface of the metal substrate is enhanced to no less than 13% or no less than 20%. At the interface between the metal substrate and the conductive film, a Cr oxide layer is formed on the surface of the metal substrate, in order to enhance the anticorrosiveness in the operating environment of the battery (i.e., about 80° C. in a saturated vapor).
However, the separators which are obtained by these methods still have less than sufficient anticorrosiveness.
Other than the above, there have also been proposed separators in which a plating of a metal film, e.g., a platinum group element or gold, is provided on a stainless steel. Since an oxide coating (passivation coating) is generated on the surface of the stainless steel, in which the Cr contained in the steel has bound to the oxygen in the atmosphere, a good anticorrosiveness is obtained but there is a large contact resistance. This is not usable as a separator material as it is. Accordingly, it might be conceivable to coat the surface of the stainless steel with a precious metal which excels is both anticorrosiveness and electrical conductivity. However, adhesion between a passivation coating and a metal film is very poor, and therefore it is very difficult to directly form a metal film on the surface of a stainless steel. Therefore, a method has hitherto been performed in which, after completely removing the passivation coating by etching or the like, an underlying plating layer which contains a metal such as Ni is formed as necessary, and then a plating of a precious metal is provided.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-162479
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2002-313355
[Patent Document 3] Japanese Laid-Open Patent Publication No. 10-68071